1. Field of the Invention
This invention relates to apparatus and methods for forming hollow articles of hardenable plasticized materials, and particularly to apparatus and techniques for forming such articles by blow-molding.
2. Discussion of Reported Developments
The production of hollow articles such as containers, by the blow molding of plasticized materials is an art that has been practiced for many years; glass is a material that has been successfully blow-molded on a commercial scale. In apparatus that has been used for forming glass containers, a gob of molten glass is placed in a parison mold and a plunger is pressed into the glass to form a parison that is supported in a neck mold. Then the neck ring, with the parison attached, is transfered to a blow molding cavity, where a blowing head inflates the parison into the shape of a finished container.
After the advent of synthetic thermoplastic polyhydrocarbon materials, came efforts to produce hollow containers from such materials. In one of the earliest commercial efforts, extrusion blow molding, which began in the 1930's, a tubular parison is extruded and thereafter the softened parison is placed in an opened split blow mold that, when closed, clamps both ends of the tube together. Various means are utilized to inflate the tube while it is inside the blow mold, to form a finished object. This method has several drawbacks, a primary one being that the amount of material consumed, for each container made, is relatively high because the portions of plastic material that are pinched off from each end of the tube by the blow mold are lost. These portions are either discarded or are reground and remelted for subsequent reuse. These factors have an adverse impact on materials and processing costs. Materials costs are an especially important factor now because rapidly rising petroleum prices have forced up the costs of the thermoplastic resins that are commonly used in container production.
A second commercially-practiced process for making hollow plastic articles is injection blow-molding. In this process, a preform is formed on a preform core by injecting plasticized material under high pressure into an injection cavity in which the core has been placed. Such systems avoid the loss of the pinched-off portions of material that are inherent in the extrusion blow molding process, provide better control of article weight, and yield a better neck finish on the article, but in turn have other drawbacks.
First, the machines for carrying out this process are costly. The basic injection press must be capable of developing at least several tons of pressure and such presses are expensive to make. Also, the cost of tooling (i.e., cores, blow cavities, neck rings, etc.) for these machines is high, because it must be made to very close tolerances so that the formation of flash on the parison is avoided. Flashing occurs when the resin, which is injected under high pressure, is forced between mating parts of the tooling and is undesirable because it is carried over to the finished container, with the result that the container is either discarded or must undergo additional processing to remove the flash; the amount of material used to form each container is also unnecessarily increased. Such tooling can easily be rendered useless by slight damage to the tools resulting from accidents or mishandling.
In addition, to provide a small cross section of material that is easily severed after the injection step, the plasticized material is injected through a very small gate or opening in the injection cavity. In order to obtain a sufficient flow of resin through the gate, the resin must be heated to a point at which the viscosity is relatively low and it must be subjected to high pressure, usually on the order of 15,000 to 20,000 psi or higher. When injection molding relatively stiff resins such as high nitrile barrier resins (the materials that are currently considered in the forefront with respect to containers for pressurized fluids, for example, carbonated beverages), injection pressures in the range of 25,000 psi are used. These high pressures cause the resin to flow through the gate at high velocity and this generate additional heat that raises the temperature of the resin above its melt temperature. This can cause deterioration of thermo-sensitive resins and results in visual blemishes, such as local opacity or discoloration, or mechanical blemishes, such as locally reduced wall thickness or voids, in the finished product. The rise in the temperature of the resin also increases machine cycle time because additional time is necessary to reduce the temperature of the preform to an optimum blow molding temperature. Another undesirable result of the use of high injection pressures is that it induces an uneven stress distribution in the preform, especially in the part of the preform near the gate, this leads to difficulties in inflating the preform evenly and also to weakened portions in the finished article.
Injection blow molding also has limitations with respect to the maximum size of container that can be economically produced. Machine cycle time is lengthened because the relatively larger volume of resin necessary to form the larger preform must be injected through the very narrow gate into the injection cavity. Moreover, injection presses capable of developing sufficient injection pressure over the relatively larger area involved are very costly.
Thermo-forming is another container production process that has been used commercially. In the more conventional thermo-forming apparatii that have been used, a sheet or web of thermoplastic material is drawn across the mouth of an article-forming cavity. A plunger forces the thermoplastic material into the cavity, sometimes aided by a vacuum drawn in the cavity or by blowing air introduced through the plunger. This process has serious drawbacks from the standpoint of material utilization, because a major portion of the web from which the article is drawn is not used and is discarded as scarp.
U.S. Pat. No. 3,602,946 discloses an improved thermoforming technique in which a pad of thermoplastic material from which the article is to be formed is molded under low pressure, the periphery of the pad being received in a transfer ring. A thin-walled article is formed from the pad of thermoplastic material by a plunger that forces the material into a forming cavity. Even by this process, however, a significant portion of the pad of thermo-plastic material remains adhered to the transfer ring after the article is formed, and this material represents waste that is not used in the formation of the article.
Efforts have been made to mold preforms using low pressure molding techniques. In one type of equipment, a ram is used to introduce a quantity of plasticized material, through a relatively large opening, into a preform cavity that contains a preform core. Examples of this arrangement are illustrated in U.S. Pat. No. 3,172,929 to Santelli and U.S. Pat. No. 3,170,871 to Ninneman et al. It should be noted that in these arrangements, the ram forms a portion of the bottom wall of the preform cavity; if the volume of the charges of plasticized material is not very closely controlled, preforms having differing bottom wall thicknesses, and thus differing blowing characteristics, are produced on each cycle, with a resulting non-uniformity of the finished articles.
Other attempts have involved compression molding the preform by pressing a preform core into a charge of plasticized material placed in a preform cavity. See U.S. Pat. No. 3,337,910 to West and U.S. Pat. No. 3,375,533 to Criss. It is believed to be very difficult to produce fully packed preforms having uniform blow molding characteristics by these means. If attempts are made to make the volume of the charge equal to the volume of the preform mold space, on a statistical basis, some of the charges will be of lower volume and an incomplete preform will result. If the cavity is overcharged by a volume of material greater than that of the mold space, then the incidence of flashing or uneven wall thickness of the preform is likely to increase.